Most of all is body language. If he faces you while talking, or sitting down even, and makes direct eye contact, there's a sign he's hooked. Also when he tends to look at you from the corner of his eye, which in that situation you should return that gaze the same way, but add a smirk and raise your eyebrows as if your silently saying, "yes? what are you looking at?" in a flirty way.

TIP! When you talk to the crush, tilt your head a little to your right if your right handed, and left if your left handed, and smile lightly, this tells him you're interested in what he has to say, and tells him you like to hear about him.